problem in welding vertices, a beginner
Maybe you've chosen the wrong option from the Merge menu?
Make sure, of the selected vertices, the ones you want to be merged are closer to one another than to any others, and Merge &gt; By Distance

Or, if that's too fiddly, (and you want the merged loop to be centered to the ones above and below) select the edge-loops you want to be merged, and Ctrl E Edge Menu &gt; Bridge Edge Loops, with 'Merge' checked in its options.